Langmuir 1994,10, 1615-1617
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Polymerization of Silicic Acid in a Collapsed Lamellar Phase M. Dubois* and B. Cabane Equipe mixte CEA-RP, Service de Chimie Mol&culaire,C.E. Saclay, 91191 Gif sur Yvette cedex, France Received November 15, 1993. I n Final Form: February 8, 1994@ The binary system didodecyldimethylammonium bromidelwater has two lamellar phases at room temerature: a swollen lamellar phase with 100 A thick water layers and a collapsed lamellar phase with 6 thick water layer. Silicic acid monomers have been dissolved in the water layers of the collapsed phase and condensed to give two-dimensional silica polymers.
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Introduction Polymerization reactions usually produce random objects with structures that are determined by connectivity alone. Examples are linear homopolymers, for which the solution configurations are close to random walks, and branched polymers, which tend to have bushy, irregular structures. Structures which are more ordered in space, e.g. periodic or anisotropic, may be obtained through the polymerization of molecules which self-assemble before reacting which each other: examples are the polymerization of liquid crystal molecules' or the polymerization of surfactant vesicles.2 However, this approach is limited to the polymerization of very specific monomers. Therefore, it is of interest to find out whether a more general route could be found to produce nonrandom structures through the polymerization of common monomers. The first step would be to perform the polymerization inside a host structure which would act as a template. Then the polymer and the template could be kept together, if this is suitable for the desirated applications, or, in the next step, the template would be destroyed, if it is intended to recover the polymer alone. The first step has been performed mainly with microemulsions, where the polymerization has been performed within the continuous oil phase3 or within the water pools4 of the microemulsions, or in the water layers of a lamellar phase;5 the second step has been used to synthesize mesoporous structures through the polymerization of silica within the honeycomb water lattice of a hexagonal mesophase.6 In previous work' we followed the first approach with a template made of the surfactant bilayers of a lamellar mesophase. The condensation of silicic acid monomers within the water layers of this phase produced silica polymers which were adsorbed on the'surfactantlwater interfaces. In that respect, an oriented polymerization reaction was performed. However it was not possible to obtain anisotropic silica/surfactant composites in these conditions. Indeed the silica polymers had a tendency to segregate out of the lamellar phase, in which case they lost the anisotropy and periodicity of the original lamellar Abstract published in Advance ACS Abstracts, April 1, 1994. (1)Laversanne, R. Macromolecules 1992,25, 489-491. (2) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem. Int. Ed. Engl. 1988, 276, 113. (3) Yamauchi, H.; Ishikawa, T.; Kondo, S. Colloids Surf. 1989,37,71. (4) Osseo-Asare, K.; Arriaga, F. Colloids Surf., in press. (5) Friberg, E.; Zhuning, Ma J. Non-Crys. Solids 1992, 147 & 148, 30-35. (6) Kresge, C.; Leonowicz, M.; Roth, W.; Vartuli, J.; Beck, J. Nature 1992,359. (7) Dubois, M.; Gulik-Krzwicki, Th.; Cabane, B. Langmuir 1993, 9, 673. @
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phase. This segregation was blocked when the lamellar phase had a texture made of spherical liposomes, but then the resulting material was not anisotropic either. It is interesting to figure out why segregation was such a problem. First, segregation was promoted by the very strong interaction between silica polymers and surfactant headgroups. Indeed, through this interaction, spatial fluctuations in the concentration of silica may disrupt the bilayer structure. Second, in the case of flat bilayers, segregation was facilitated by the fluidity of the lamellar phase. In this note we report how the same polymerization reaction may be performed in a different region of composition of the same surfactant/water system, where these problems are avoided. The key to this progress was the use of a mesophase which was lamellar but contained very little water;s this prevented the large concentration fluctuations which could lead to segregation.
Lamellar Phase The temperature-concentration phase diagram of the binary system DDAB-water is shown in Figure 1. At room temperature there are two lamellar phases, which are labeled L, and La'in Figure 1. The L, phase is the usual lamellar phase, made of surfactant bilayers with liquid chains, and water layers which are a t least 100 A thick.8 The area per polar head in this phase is 67 A2. The La' phase is a collapsedlamellar phase, also made of surfactant bilayers with liquid chains; in this phase the water layers are only 6 A thick, and the area per polar head is the same.8 The ranges of stability of these two phases are separated by a two-phase region; at room temperature this region extends from compositions containing 30 % DDAB to compositions containing 75% DDAB. The reasons for the phase separation have been discussed in a previous publi~ation.~ At lower temperatures there is a phase where the chains of the surfactant molecules are in a rigid configuration; this phase is labeled L,+ The temperatures for chains melting have been measured by differential calorimetry (DSC4 Perkin-Elmer); they are 17 "C for the transition from Lg to L, and 22 "C for the transition from Lg' to La' phases. The liquid state of the surfactant chains in the L, and La'phases is also confirmed by X-ray diffraction spectra, which show a broad, diffuse band in the range of interchain distances (Figure 2). (8) Zemb, Th.; Gazeau, D.; Dubois, M. Eur. Phys. Lett. 1992,21,7 759. (9) Zemb, Th.; Belloni,L.; Dubois, M.; Marcelja, S.Prog. ColloidPolym. Sci. 1992,89, 33-38.
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Figure 1. Partial (C,-T) phase diagram of degased solutions of DDAB in H20; L,' is the collapsed lamellar phase with a periodicity of 32A includingthe thin water layer (6 A). L, is the swollen lamellar phase which can be diluted with water to 3%
DDAB content. The two-phase region between the lamellar and the isotropic phase located at higher temperature is too narrow to be experimentally determined.
At higher temperatures the range of phase separation shrinks, and above 75 "C there is no separation between the swollena and collapsed phases. It has been shown that the uppermost boundary of the separation region is a critical point and that the Laand L/ phases are a single thermodynamic fluid phase. This continuous path is of practical importance, because it allows samples to be prepared in conditions where it is easy to ensure homogeneity of composition before cooling to a more rigid phase. Polymerization Reactions Silicic acid monomers polymerize spontaneously in water. The reaction is endothermic, it is slowest at the isoelectricpoint of silica (pH 3) where it takes a few hours to reach the gel point in a solution containing 10%silicic acid in water. The reaction is accelerated by the presence of DDAB in a swollen lamellar phase containing water, DDAB (10%of the total weight), and silicic acid (18%of the total weight) the reaction is accelerated by a factor 4; in a collapsed phase containing 80% DDAB and a 2 % silic acid the acceleration factor is 8. This fast reaction rate makes it difficult to mix the components and obtain a homogeneous lamellar phase before the polymerization takes place. The difficulty is particularly significant for the collapsed lamellar phase, which is quite viscous. With pure DDAB/water systems it is convenient to heat the sample above 120 "C, where the lamellar phase is replaced by a fluid micellar phase (Figure l ) , mix at high temperature, and then cool down into the collapsed lamellar phase. This is not possible for samples containing silicic acid monomers, because poly-
Figure 2. X ray diffraction pattern from a collapsed lamellar phase of DDAB and water: 80%DDAB, 20% water, T = 20 O C . The instrument was a Guinier camera with linear (vertical) collimation. The intense set of rings near the center is the first diffractionorder;the next twoorders are also visible. The diffuse band at wider angles is the diffraction band corresponding to distancesbetween neighboring aliphatic chains in a liquid state.
merization is too fast. For this reason samples were prepared by mixing as well as possible at room temperature and using high-speed centrifugation to remove all air bubbles.
Structures of Polymerized Samples Homogeneous, transparent lamellar phases were obtained with samples containing 75% DDAB and 2.5% silicic acid. Upon polymerization the samples remained transparent and birefringent, indicating that the lamellar organization had been preserved and that no segregation had taken place. This was confirmed by X-ray diffraction; the scattering curves showed a peak at the repeat distance of DDAB bilayers and no scattering next to the beam, where the effects of segregation would have been observed (Figure 2). Moreover the peak of the mixed lamellar phase
Langmuir, Vol. 10, No. 5,1994 1617
Polymerization of Silicic Acid
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--.-... ...,.__._.... 0 0.18
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Figure 3. Small-angleX-ray scatteringfrom a collapsed lamellar phase containingpolymerized silica: horizontal scale, scattering vector (A-1); vertical scale, scattered intensity (absolute units). The peak position corresponds to a period of 30 A; the same period is obtained from the first diffraction in Figure 2. The higher peak is from a sample aged from 1 month at room temperature,the lower one from a sample aged from 15 months at T = 50 "C. The peak located at lower Q (to the left) is from a pure DDAB/water mesophase.
had the same width as that for a pure DDAB mesophase, indicating that the quality of ordening was not changed by the polymerization of silicic acid. Therefore the samples consisted of (water + silica) layers sandwiched between surfactant bilayers. This is in contrast with our previous observations on the swollen lamellar phase, where the polymerization of silicic acid caused substantial disorganization of the bilayers (revealed by a broadening of the peak) and segregation (revealed by scattering next to the beam). Accordingly, it may be concluded that the bilayers of the collapsed phase are sufficiently stiff to resist the stresses caused by polymerization. It is also necessary to examine whether the silica layers are the true two-dimensional objects, i.e. whether the silica polymers are completely confined in the layers. This question may be answered by examining the mechanical properties of the collapsed lamellar phase; samples with polymerized silic are viscous, but they still flow. This would not be possible if the silica layers were connected to each other, which would make the samples behave as gels. Therefore it may be concluded that the layers of silica are independent of each other.
Extraction of Silica Polymers Now we turn to the second step mentioned in the introduction, i.e. destroying the template in order to recover the two-dimensional polymer alone. Here the
difficulty originates from the reactivity of the silica polymers. Indeed these branched polymers have reactive silanols as side groups and chain ends. When polymerization occurs in a solution, these polymers react with each other until a three-dimensional network is formed.10 Similarly, silica polymers contained in the water layers of a surfactant mesophase may condense together if the surfactant is washed out. In order to stop the reaction between silica polymers, we used a dating agent, hexamethyldisilazane (HDMS), which caps the reactive silanol groups. HDMS was dissolved in toluene, and the lamellar phase with polymerized silica was swollen with this oily solution. The aim was to increase the separation of the silica layers, make them unreactive and hydrophobic, and finally disperse them in the oil. However, instead of swelling the lamellar phase, the addition of toluene and HDMS caused the samples to separate into an oil solution and water. If the addition of toluene and HDMS was performed after a short polymerization time (2 days), then it was observed that the oily phase contained some birefringent filaments, presumably made of silica with DDAB bound to it (DDAB alone forms an isotropic, homogeneous solution in excess toluene). If the addition was performed after a long time (several months), then all silica was found segregated as opaque polymeric lumps, also with some bound DDAB.
Conclusion Layered materials made of silica polymers separated by surfactant bilayers can be produced through the condensation of silicic acid in the water layers of a collapsed lamellar mesophase. This structure is stable as long as the surfactant template is kept in place; it could serve as a host for binding oriented molecules, for instance, dyes for second harmonic generati0n.l' If the surfactant template is destroyed, then the silica polymers collapse or aggregate together into three-dimensional lumps. Attempts to prevent this aggregation of two-dimensional polymers have not been successful. This is not surprising, sincethese polymers must be quite flexible; thus a complete capping of all reactive side groups would have been necessary to prevent aggregation or collapse.
Acknowledgment. Part of this work was done with Delphine Charvolin. It is a pleasure for us to acknowledge her contribution to the early experiments on polymerization in collapsed lamellar phases. (10)Dubois, M.; Cabane, B. Macromolecules 1989,22, 2526-2533. (11)See for instance Severin-Vantilt,M. M. E.; Oomen, E. W. J. L.J. Non-Cryst. Solids 1993, 159, 38.
